Electrical signaling system



April 24, 1951 r. B. D. TERRONI ET AL 7 2,550,312

ELECTRICAL SIGNALING SYSTEM Filed March 5,1947 s Sheets-Sheet 1 AAL I ME 3 I CL DBFI F GA V LCU LE RDF N N DE DBF2 DBF3 INVENTORS.

PR TESEO BRUNO DANTE TERRONI JOHN ERNEST THOMAS PARKER ATTO R NE Y April 24, 1951 T. B. D. TERRONI ET AL 2,550,312

ELECTRICAL SIGNALING SYSTEM 'F' iled March 5, 1947 3 Sheets-Sheet 2 MAX INVENTORS. TESEO BRUNO DANTE TERRONI JOHN ERNEST THOMAS PARKER M'TO RNEY April 24, 1951 T. B. D. TERRONI ET AL ELECTRICAL SIGNALING SYSTEM Filed March 5, 1947 FIG.4

3 Sheets-Sheet 5 NVENTORS.

l TESEO BRUNO DANTE TERRONI JOHN ERNEST THOMAS PARKER ATTORNEY Patented Apr. 24, 1951 ELECTRICAL SIGNALING SYSTEM Teseo Bruno Dante Terroni and John Ernest Thomas Parker, Liverpool, England, assignors to Automatic Telephone & Electric Company Limited, Liverpool, England, a British com- Application March5, 1947, Serial No. 732,574 In Great Britain March 6, 1946 12 Claims.

The present invention relates to electricalsig nalling systems employing modulated carrier currents for the transmission of information over a transmission circuit and is more particularly concerned with systems having automatic gain control arrangements operative in response to changes in the attenuation of the transmission circuit. 1

In the usual automatic gain control circuits, if a sudden change of more than a predetermined value takes place in the line attenuation, no automatic gain control occurs but an alarm signal is given to attract the attention of the supervising personnel who will attend to the equipment on the assumption that a fault has occurred. The predetermined value of the changes permissible before an alarm is given may, however, be reacted in conditions other than fault conditions. For example, abnormal weather conditions may often give rise to sudden changes which may be greater than the predetermined value but which do not require supervision. Thus it is found that due to heavy snowfall a considerable amount of line loss will have to be compene sated by increase of gain but such increase should be allowed to take place automatically without calling a supervisor.

One of the objects of the present invention is to provide a system using automatic gain con-' trol which is more flexible than previous systems in that it is able to discriminate between changes due, for instance, to abnormal weather conditions and those due to fault conditions. I

According to one feature of the-invention, in response to a change in the attenuation of the transmission circuit, the automatic gain control arrangements are adapted to initiate a predetermined maximum correction which is less than the total maximum correction of which the arrangements are capable.

According to a further feature of the invention, in response to a change in the attenuation of the transmission circuit, the automatic gain control arrangements are adapted to initiate a predetermined maximum correction and to effect the required correction if it is less than the predetermined maximum or to give an alarm if the required correction is greater than the predetermined maximum.

According to another feature of the invention, in response to an abnormal change in the attenuation of the transmission circuit, the automatic gain control arrangements are adapted to effect a predetermined maximum correction and then determine whether the correction still to be made I exceeds a, predetermined amount.

According to yet another feature of the invention, in response to an abnormalchange in the attenuation of the transmission circuit, according to the setting of one or more variable controls, the automatic gain control arrangements, either are prevented from applying a correction and an alarm is given or are capable of effecting a correction of a predetermined maximum amount in one or both directions.

According to a further feature of the invention, in response to a change in the attenuation of the transmission circuit, the automatic gain control arrangements are adapted to initiate a predetermined maximum correction which is effected by the step-by-step operation of an automatic switch under the control of a timing device which is so adjusted that the switch is capable of taking such a number of steps as to provide the predetermined maximum correction.

The invention is described with reference to, a terminal station in a carrier current'transmis-r sion system but on a long line,particularly of the openwire type, it may with advantage be employed at one or more intermediate repeater stations.

The invention will be better understood from the following description of one example taken in conjunction with the accompanying draw ings comprising Figs. 1 to 4 of which Fig. lshows a block schematic of part of the terminalequip ment of a three channel carrier system while perm-3.

Figs.- 2 to 4 when placed side by side with Fig: 2 on the left show the complete circuitdiagram of the automatic gain control arrangements. f

Referring to Fig. 1, outgoing modulated carrier signals are transmitted to the carrier line CL by Way of the transmitting directional filter TDF which connects with the three modulated equ p:

ments on lead ME. p L

As regards reception the incoming modulated carrier signal is extended fromr-the carrier-line CL to the receiving directional filter RDF whence the side band frequencies are passed to the direc'-' tional line equaliser LE which compensates for the relatively higher losses occurring at high fre-% quencies. The signal is now fed to the level con-g trol unitLCU which is part of the gain control arrangments. From here the signal is applied to the group amplifier GA and then via the atten'ug ation pad P to the demodulator band-pass filters The latter connects with three 'de: modulating equipments over leads such as. DEL-1}."

For the purpose of automatic gain control,

the incoming signal is tapped off the group amplifier GA and fed to the pilot receiver PR, where a band-pass filter selects the pilot frequency. The pilot frequency is amplified and then rectilied to give a direct current voltage proportional to the amplitude of the pilot frequency. It will v the contact voltmeter type, one of which closes its contacts due to a change in D. C. voltage corresponding to 1 /2 db. change in the line attenuation and the other which closes its contacts due to a :4 db. change in line attenuation.

The db. relay controls a stepping switch (hereinafter referred to as the pad control switch) with a forward and reverse drive which compensates for changes in line attenuation to within half a decibel over a rang of ilO db. from normal by introducing, as previously mentioned, loss pads progressively increasing or decreasing in db. steps.

The 4 db. relay is used for gain control purposes and for instituting alarm signals, to be referred to later, when large changes in line attenuation occur suddenly.

An initial delay of the order of 6, 12, 18 or 24 seconds precedes the operation of the pad control switch to effect line compensation. The initial delay is necessary to prevent operation of the equipment by momentary line attenuation changes. The delay time is variable to cater for the condition where the arrangements are provided at a repeater, station or stations in addition to the terminal stations'on a long line. If the time delay is fixed in such a case then with a change of attenuation in, say, the first line section, the arrangements. in subsequent line sections would commence operation at the same time. If the time delays of the successive AGC arrangements are staggered, it can be arranged that the AGC of the first line section will become operative first to correct the overall gain before the AGC as the succeeding sections begin to operate. Such an arrangement leads to minimum AGC operation and more uniform conditions along the line.

An alarm signal is given by the 4 db. relay under one of the following conditions; according 1 to the setting of two preset controls:

1. With a sudden change of :4 db, or more in the line attenuation the pad control switch stepping circuit is disconnected, the delay circuit reset and after the delay interval (equal to initial delay time) the alarm is given.

2. As in the previous case but the alarm is given when the gain of the system is required to be increased by 4 db. or more. In the direction of decreasing gain, the pad control switch is permitted to step to its minimum position and a limit alarm is then given if a further change is required.

3. With a sudden line change of :4 db. or

more, pad control switch is permitted to step (say) 8 times, i. e. a 4 db. change, and if the level I be understood that any change in the line attenuis not then corrected, the switch is stopped and the alarm given.

4. As in the previous case but the alarm is given in the direction of increasing gain only; in the direction of decreasing gain the switch is permitted to step to its limit position.

5. With a sudden line change of :4 db. or more, pad control switch is permitted to step (say) 6 times, i. e. 3 db. if then the level is 4 db. or more from normal an alarm is given, but if the level is 4 db. or more from normal the switch is permitted to step until the level is again normal.

6. As in the previous case but the control and alarm operate in the direction of increasing gain only, while in the direction of decreasing gain and pad control switch is permitted to step to its limit position.

7. With any line change, the pad control switch is permitted to step only (say) 8 times, i. e. a change of :4 db., and if the level is not then normal an alarm is given.

'8. As in the previous case but the control and alarm operate in the direction of increasing gain only, while in the direction of decreasing gain, thev pad control switch is permitted to step to its limit position.

The facility whereby in the direction of decreasing gain the pad control switch is permitted to step to its limit position is useful when weather conditions are known to be improving in which case the line attenuation will decrease steadily by possibly 8 or 10 db. to normal so that alarms constitute an unnecessary demand on supervision.

A detailed description will now be given of the circuits shown in Figs. 2, 3, and 4, Fig. 2 showing the circuit of the pilot receiver PR while Figs. 3 and 4 show the circuit of the level control unit LCU.

Pilot frequency receiver PR Referring to Fig. 2, the high tension supply is connected to leads l0 and II which are positive and negative potential respectively. The incoming signal is tapped off the group amplifier GA and applied to primary winding of transformer TRA and passed by the tapped secondary winding to the pilot frequency selecting filter. The transformer has a step-down ratio of 11 to 1 to reduce the bridging loss of the circuit in the transmission line.

The selecting band pass filter has two capacitycoupled, temperature-compensated, series resonant circuits comprising inductance Ill/capacity Gland inductance L2 /capacity C2 tuned to the particular pilot frequency. Variable condensers V0! and V02 provide fine adjustment for tuning. A feature of this type of circuit is the high degree of selectivity, while at the pilot frequency the response curve is flattened or double-humped, depending upon the ratio of the value of the tuned circuit condensers to the coupling condenser C3. In this particular embodiment, however, the ratio or coupling factor is so chosen as to give a single fiat-topped response curve at the pilot frequency with an attenuation of at least 20 db. at 250 cycles above or below the pilot frequency.

The pilot frequency voltage developed across inductance L2 is passed to the grid of the amplifying valve VA where the signal is amplified and fed to the meter circuit via the output transformer TRB in the valve anode circuit. A small amount of negative feedback is produced in the amplifying stage by the bias resistor YC in the cathode circuit, the amount of feedback being sufiicient to stabilise the gain of the unit with varyingipower supplies, and to maintain the gain within approximately /2 db. for different valves;

The rectifier MRA in the secondary circuit of the anode transformer 'TRB converts the amplified pilot frequency signal into a direct current signal which actuates two sensitive relays SL and tics of the amplifying stage, or affecting the D. C. response characteristic of the rectifier and meter circuit. l i r 1 When the arrangement is being calibrated initially the incoming band-pass filter is tuned tothe actual pilot frequency and then the gain is adjusted 'until the level indicating meter LlVl reads 0 db. Under this condition the current developed will be of the order of, say.1,30 micro amps. and the armatures of the sensitive relays SL and SP would lie centrally between their respective contacts.

Any change in the line attenuation of the carrier system alters'the levelof the incoming pilot frequency and produces a proportionate change-of the D. C. voltage-in the meter circuit.

The level indicating meter calibrated in decibels indicatesthe value of the change and one or both of the sensitive relays SL and SP may be actuated. The function of these relays will be described in detail later but before proceeding with the description relating to compensation for line attenuation, an explanation of the timedelay circuit element of the level control unit is necessary.

Time-delay circuit element As previously stated it is necessary to precede any correction of the line attenuationby a predetermined delay, hereinafter termed the initial the lower plate in the circuit and negative on the upper plate) through contacts DS! in their normal position by the voltage developed across the tapped potential divider which comprises resistance YL, variable. resistance VR i and disc ARAconnected in series. Disc ARA may comprise non-linear discs or elements of like formation ofthe carborundum-ceramic type such as described inthe United States Patent 2,292,977, page 3, column 2, lines 21 through 37.

When relay DS is operated (as later explained) the polarity of the potentials applied to condenser Cl is reversed and it'will first discharge and then re-chargeto the opposite polarity at a rate determined by the series resistances VRZ and In addition, contacts DS2 of relay DS connect the condenser C1 in series with resistor YM, between the control'grid and the cathode of the 6 coldcathode neon tube NA, and at contacts DSI, connect high tension positive battery to relay PA in series with resistance YJ, in the anode circuit of the tube.

When the voltage across the condenser reaches the breakdown voltage of the tube (say 75 volts) the tube strikes and current passes in the anode circuit to operate relay PA. The utilisation of the contacts of relay PA will be explained later.

Predetermined initial delay times over a'wide range are available by appropriate adjustments on the variable resistances VRZ and VRG, the preferred values of the delay being 6, 12, 18 or 24 seconds.

The circuit is prepared for the stepping interval delay by the operation of relays DS and SC. Contacts 8C4 disconnects the variable resistance VRZ and replaces it by VR3 which is preset to give the necessary discharging and charging rate for condenser C! and the consequent delay in the operation of relay PA.

' The pad control switch (PC) This is a selector switch with two driving magnets PCIM and PCDM for forward or reverse stepping of the wipers. In its preferred form the PC switch has 41 wired band positions, each position'connecting a loss pad such as AP! in the receiving path of the carrier system. The pads range from 0 to 20 db. in db. steps, and, with the lineattenuation at its normal value, the PC switch is'positioned to bring, the 10 db. pad into the line. Provision is therefore made for compensating for up to 10 db. variation of line attenuation in db. steps.

Operation with 1 db. increase in line attenuation Assume that the line attenuation has increased by 1 db. from the normal value. This reduces the level of the received pilot frequency by the same amount, the pilot receiver thereby causing the SL relay to close its SLI (L) contacts.

Positive battery is then extended from the operated contacts AAI (Fig. 2) of the pilot receiver anode alarm relay AA by the SLI (L) contacts of relay SL through relay IG andmagnet PCDM. Relay IG only operates and at 1G3 completes a circuit for relay BS from positive battery, AAI, KAGC'O, IG3, LA2, S03, ADZ, Winding of relay DS, negative battery. At operated contacts BS! and D82 the time delay circuit functions as previously described and after the predetermined delay period relay PA in the anode circuit of the neon tubeoperates and at PAI operates relay SC. Relay SC locks over S03 to the positive battery at AAI, and releases relay DS. Relay DS in releasing at DSl normal releases relay PA and resets the timing circuit.

With contacts PAI in their normal position a circuit i completed for the slow operate relay A from positive battery, PAI, SCI, Bl, winding of relay A, YH to negative battery. At contact Al positive battery is extended to the increase the contacts of relay SL remain closed holdin relay'IG, sincean increase of 1 db. is-required.

ThePC switch is stepped once again by the action of relays A and B, to bring in circuit the 9 db. pad. The level of the pilot frequency will now be normal again, thereby causing relay SL to restore and release relay IG. Relay IG at normal contacts. IG3 releases relay SC to disconnect the switch stepping circuit. 7

Should the line attenuation decrease by 1 db. then the level of the incoming pilot frequency would be increased by the same amount, the pilot receiver, causing relay SL to close its SLI(H) contacts. In this case relay DS is operated direct from the SL contacts, and the PC switch is subsequently stepped in the direction of decreasing gain by the interaction of relays A and B, and by the energisation of the decrease magnet of the PC switch (PCDM) through the normal contacts of relay IG.

Limit alarm If and when the PC switch reaches either its minimum or maximum position due to changes in line attenuation and a further pad change in the same direction becomes necessary, further stepping is prevented, andan alarm is given after a time interval equal to the initial delay period.

Thus if the PC switch is in its limit position and the SLHL) or SLHH) contacts are closed indicating that a further change is necessary in the same direction, relay LA operates.

Relay LA at LAZ disconnects relay DS or SC if the latter is operated, causing the timing circuit to be reset and the PC switch to be prevented from stepping further by the opening of its stepping circuit at SCI. At LAI a circuit is completed for the slow-operate relay AD which in operating re-operates relay DS.

Operated contacts of relay DS cause the timing circuit to function, and after the delay time relay PA operates.

Positive battery is connected via PA! and ADI to lead it to give an external alarm, and to the pilot alarm lamp PAL on the "level control unit.

The maintenance oflicer may disconnect the external alarm by operating the alarm cut-off key KACO, but the pilot alarm lamp remains lighted so long as the alarm condition persists.

When nowthe line attenuation changes to require the PC switch to step in the opposite direction, relay SL changes-over closing its contact opposite to the one operated in the alarm condition. Relay LA is, therefore, released, and. opens the circuit of relays DS and AD. Relay DS releases and remains normal until relay AD (slow release) returns to normal. During this interval the time delay circuit is reset and relay PA released.

With PA normal the pilot alarm lamp is extinguished. Positive battery at AAI is extended to the external alarm circuit lead 15 by AD3 and PAZ normal and KACO operated. The pilot clear lamp PCL is also lighted from the same positive battery to call the attention of the maintenance ofiicer, who, on restoring KACO disconnects the external alarm and extinguishes the pilot clear lamp.

Operation of alarm circuit with sudden large Zine attenuation changes Case 1. For this the manually operated selecting switch AS is set in position I, and the link DAL is inserted.

If, now, the line attenuation changes by less than i 4 db. the level is corrected under the control of the /2 db. relay SL as previously explained. When the line changes suddenly by i 4 db. or more, relay SP assumes control of the pilot 5 receiver and closes either its SPI (H) or SPHL) contacts (for a decrease or increase in line attenuation respectively). Positive battery at AAI is extended by way of the closed contact relay SP to operate relay LA. As described previously relay LA releases relay DS or SC to prevent the PC switch from stepping further, and controls the time delay circuit which after a delay interval equalto the initial delay, gives the alarms.

Case 2.In this case the selecting switch AS is set in position I, and the link DAL omitted so that the alarm condition described under case i functions only when the line attenuation increases by 4 db. or more (requiring an increase in gain of the equipment), and the pilot receiver causes the SP! (L) contacts to close. In the other direction i. e. when the line attenuation is decreased by 4 db. or more, although the SPHH) contacts are closed they perform no function, and under the control of the db. relay SL the PC switchis permitted to reduce the gain of the circuit and step to its minimum limit (MIN contact) position if necessary.

Case v3.In this case switch AS is set in position 2* and the link DAL also inserted. When there is a line change of :4 db. or more relay SP closes its contacts one way or the other. At the same time, the /2 db. relay SL closes one of its contacts to start the time-delay equipment. At the end of the initial delay period relay PA operates to operate relay SC, and commence the stepping of PC switch.

When relay SC operates, the time delay circuit is reset by the restoration of relay DS, but, at the same time, relay AD is operated by the 40 positive battery from AAI, the closedSP relay contacts SPI, wiper AS2 in position 2, CS2 and through the winding of relay AD to negative battery. Relay AD looks from positive battery via AAI, KAGCO, AD3, wiper ASI in position 2, DAL, wiper AS2 in position 2, SC2, winding of relay AD to negative battery. Relay DS is reoperated by relay AD. The time delay circuit is thus brought into operation again, but the normal charge control resistance VR2- is replaced,

through contacts of relay SC, by another variable resistance VR3 preset to give a time delay interval equal to the time taken by the PC switch to step say 8 times. At the end of this period, i. e. the PC switch having changed the gain by 4 db., relay PA is operated to remove the positive from the stepping circuit, and to give the alarms as previously explained.

Should, however, at the end of this interval the line level have been corrected, the V; db. relay resets, relay SC is released, and the circuit returns to normal without any alarms being given.

Case 4.In this case the selecting switch AS is set in position 2, and the link DAL omitted. This case is similar to Case 3, except that the stepping is controlled and the alarm given, as described, only when the gain of the circuit is required to be increased, i. e. when the SPI (L) contacts of relay SP operate. When the gain is required to be decreased, although the SPI (H) contacts of relay SP close, they perform no function and the PC switch is permitted to step, if necessary, to its minimum limit position.

:' Case 5."In this case the selecting switch AS is set to position 3, and the link DAL inserted.

' When the line attenuation changes by :4 db. or more, relay SP closes its contacts one way or the other. At the same time the db. relay SL closes oneof its contacts to operate relay D8 which starts the time delay equipment. At

the end. of the initial delay period relay PA operates to operate relay SC, which releases relay DS. Relay DS resets the time delay circuit. The PC switch stepping circuit is then completed. When relay SC operates, a. circuit iscompleted for the slow-operate relay AD from positive battery at AAI via SPHH) or (L). Relay AD operates relay DS which restartsthe time-delay equipment. The operation of relay PA is now predetermined ,by the stepping interval controlled by resistance VR3 which is preset to permit the P C switch to make, for instance, six

steps, i. e. change the gain by. 3 db. If at the end of, this time relay SP is still operated, indinormal, the alarm is given'and no further step ping of the PC switch takes place.

Should, however, the level be less than 4 db.

from normal after the PC switch has changed limit position (MIN) and then give the alarm.

It will be seen that with this case the range of relay SP has been increased and furthermore if the change is more than '7 db., correction to I the extent of 4 db. only is made. This condition is of value if it is anticipated that due to heavy snowfall, for example, a considerable line loss will have to be made good by increase of gain while it is undesirable to increase supervision.

Case 7. -In this case the selecting switch AS is set to position], and the link DAL inserted. With this arrangement whenever the gain of the circuit is required to be changed by more than /2 db. relay SL closes one of its contacts and relay DS operates to start the time-delay equipment. Relays PA and SC subsequently operate. Relay DS releases to reset the time delay equipment. Relay AD operates from positive battery at AAI extended via KAGCO, wiper ASl in position. 4, DAL, wiper AS! in position 4, SC2, winding of relay AD to negative battery. Relay AD re-operates relay DS which re-starts the time delay equipment now controlled by the preset resistance VR3 to allow the PC switch to step (say) eighttimes. Stepping will, however, cease if relay SC releases due to normalising of the pilot level on any one of the steps. Failing this, if the pilot level has not been compensated by the eighth step, stepping then ceases due to the operation of relay PA, and the alarms are given.

Case 8.-In this case the selecting switch is set to position 4, and the link DAL omitted. This case is similar to Case 7, when the gain of the circuit is required to be increased. When required to be decreased the PC switch is permitted to step, if necessary, to its minimum limit position (MIN) and there gives the limit alarm.

, It will be appreciated from the above descrip- 'cating that the level is still .4 db. or more from a .tion that the system provides-a very flexible control over the gain of the line. Thus in conditions of abnormal change in the weather the system may be operated as described, for example, under Cases 5 or 6 above and when a predetermined amount of line loss has been compensated the system might be switched to Case 1 or 2.

A Manual stepping The automatic gain control equipment can be prevented from functioning by operating the AGC Off key KAGCO. A lamp AGCOL associated with the level control unit glows to indicate the condition, and by means of a push key KSU or KSD the PC switch can he stepped to either increase or decrease the circuit gain.

"Pilot receiver valve jail alarm Should the amplifying valve VA of the pilot receiver fail, relayAA restores and AAI causes the anode alarm lamp AAL to glow and disconnects the positive battery from the level control relays, and the level control unit ceases to function auomatically. AA2 extends positive battery to an external alarm connected to lead I4;

Automatic gain control test The test key KAGCT is operated and inserts a pad of predetermined loss into the line. The response of pilot receiver and the level control unit would then be noted.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is .to be performed, what we desire to have protected by Letters Patent will be pointed out in the following claims.

We claim: s

1. In a carrier current transmission system, a circuit subject to changes in attenuation and over which modulated carrier currents are received, automatic gain control equipment nor mally connected with said circuit, receiving L means and control means included in said equipment, said receiving means operated responsive to a change in attenuation of said circuit for operating said control means to vary the gain insaid circuit, said operation of said control means initiating a predetermined maximum correction in gain, which is less than the total maximum correction of which said control means is capable, for each change in attenuation.

2. In an electrical signalling system, a circuit subject to variations in attenuation and over which modulated carrier currents and an auxiliary alternating current are received, said auxiliary alternating current varying in amplitude in accordance with such variations in attenuation, an automatic gain control device normally connected with said circuit, receiving means included in said device and operated responsive to a change in the amplitude of said auxiliary current for operating said device, each operation of said device causing a predetermined maximum correction in the gain of said circuit for each change in the amplitude of said auxiliary current. v

3.- A system such as claimed in claim 2. in which there is an automatic stepping switch operated responsive to the operation of said gain control device for performing said correction, and a timing device for controlling the operation of said stepping switch. .7

4. A system such as claimed in claim 2, including a means operated in response to a change in theamplitude of said auxiliary current of greater than a certain amount for preventing any correction in the gain of said circuit. 5. Asystem such as claimed in claim 2, including an alarm arrangement controlled responsive to the operation of said gain control device for operating analarm at certain times. '6. In an electrical signalling system, a circuit subject to variations in attenuation and over which modulated carrier-currents and a pilot alternating current are received, amplifying and rectifying meansnormally connected with said circuit for receiving, amplifying and rectifying said pilot alternating current whereby said variations' of-attenuation occurring in said circuit maybe detected, a plurality of compensating resistance pads,sensitive relay means operated by said amplified and rectified pilot current in response to a variation of said attenuation, coni trol relay means operated in response to the operation 'of said sensitive relay means, time delaying meansloperated in response to the operation of 'said'control relay means, an automatic stepping switch having access to said resistance pads, and means operated under control of said'delaying, means for operating said automatic stepping switch to introduce a particular one of said resistance pads into said transmission circuit to compensate for said Variation in said attenuation.

7. An electrical system such as claimed in claim 6, wherein said time delaying means'comprises an electron discharge device, a relay, a condenser, a non-linear resistance disc, a plurality of non-variable resistances, a plurality of variable resistances, and contacts controlled by said control relay.

8. An electrical system such as claimed in claim 6, including a manually operable cut-off key for preventing the automatic operation of said stepping switch, and an increasing push key and a decreasing push key each manually operable at diiferent times subsequent to an operation of said cut-off key for completing a circuit to oper ate said automatic stepping switch for introducing diiferent ones of said resistance pads into said transmission circuit to respectively increase or decrease the gain of said transmission circuit.

9. In an electrical signalling system, a circuit subject to changes in attenuation and over which modulated carrier currents and a pilot alternating current are received, a sensitive relay, receiving means normally connected-to said circuit and operated responsive to receiving said pilot alternating current for detecting variations of attenuation, said sensitive relay operated by said receiving means in response to an abnormal variation in either direction of attenuation in said circuit, control relay means operated in response to said operation of said sensitive relay, time delaying means operated in response to the operation of said control relay means for preventing further operation of said control relay means for a predetermined time interval, an alarm arrangement for operating an alarm, means for restoring said time delay means after the expiration of said predetermined time interval, means operated in response to the restoration of said time delay means for introducing a correction in said transmission circuit to compensate for an abnormal variation of attenuation in one direction, and means operated in respons to the restoration of said time delay means for operating said alarm 'incase the abnormal variation of attenuation is in the other direction. 7

" 10. An electrical "signalling system such as claimed in claim 9, i ncluding a' ma uauy settable control switch having a first and a second wiper each having a plurality of positions, a first contact on said sensitive relay connected to said first wiper, a second contact on said sensitive relay upon the directionof said abnormal variation, for r at times operating said'alarm and for at other times introducing corrections to compensate for said attenuation variations.

11. An electrical signalling system such as claimed in claim 9, including a'jumper link for connecting said first and said sensitive second relay contactsin multiple, whereby said alarm is operated in response to theoperation of said sensitive relay irrespective of the direction and value of said abnormal variation of said attenuation.

12. In an electrical signalling system, a circuit subject to changes in attenuation and over which modulated carrier currents and a pilot alternating current are received, an automatic gain control arrangement connected with said circuit comprising a receiving unit and a control unit for compensating for variations from normal of said pilot alternating current, amplifying and rectifying means included in said receiving unit for receiving, amplifying and rectifying said pilot alternating current to detect variations of attenuation occurring in said transmission circuit, a plurality of compensating resistance pads, said control unit including control relay means, a time delaying means and an automatic stepping switch, sensitive relay means also inoluded in said control unit and operated by said amplified 'andrectified pilot current in response to receipt of a variation of said attenuation for operating said control relay means, said time delaying means operated by said control relay meansfor'delaying further operation of said control relay means during a predetermined time interval, means operated responsive to the expiration of said time interval forpreparing said automatic switch for operation and for restoring said REFERENCES CITED The following references are of record in' the file of this patent:

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